1. Field of the Invention
The present invention relates to a transfer device, an image forming apparatus and a recording medium.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to a photoconductor drum (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form a toner image on the sheet.
As a transfer device used in the above-mentioned image forming apparatus, a transfer device including a first transfer member (for example, intermediate transfer belt) which bears a toner image and a second transfer member (for example, secondary transfer roller) which forms a transfer nip with the first transfer member is known. Normally, a transfer bias composed of a DC voltage having a constant value is applied to the second transfer member, and a transfer electric field is formed between the first transfer member and the second transfer member. With the transfer electric field formed in the above-mentioned manner, the toner image on the first transfer member is electrostatically moves to the sheet, and the toner image is transferred to the sheet. For the transfer devices of the above-mentioned type, various techniques for improving the efficiency of transfer of a toner image to a sheet have been proposed.
For example, Japanese Patent Application Laid-Open No. 3-132684 discloses a configuration in which a transfer bias composed of pulse waves including a first part and a second part is applied to a second transfer member, the first part corresponding to a first DC voltage capable of transferring a toner image to a sheet passing through a transfer nip, the second part corresponding to a second DC voltage which is smaller than the first DC voltage in absolute value and has the polarity as that of the first DC voltage.
In addition, Japanese Patent Application Laid-Open No. 2012-42832 discloses a configuration in which a transfer bias composed of pulse waves having alternating current components in which a transfer voltage and a returning voltage having different polarities are alternately repeated is applied to the second transfer member. In this configuration, in a recess of a sheet having a small surface smoothness, that is, a sheet having a surface having a large irregularity such as embossed paper, reciprocation of toner between the first transfer member and the sheet is caused with the transfer voltage and the returning voltage for the purpose of improving the transfer.
Incidentally, a space is formed between the recess on a surface of a sheet having a large irregularity and the first transfer member, and therefore, for example, when transfer is performed using the transfer bias composed of the first DC voltage as disclosed in Japanese Patent Application Laid-Open No. 3-132684, the transfer electric field formed in the recess is insufficient to move the toner to bottom of the recess due to the space. When toner is transferred with such a transfer electric field, the toner does not easily move to the bottom of the recess, and consequently transfer defect is caused at the recess of the sheet.
In addition, it is conceivable to increase the value of the transfer bias in order to form electric field enough to move the toner in the recess. However, when a transfer bias having an increased voltage value is kept applied to the second transfer member, electrostatic discharge is easily caused in the recess, and in addition, reverse charging of the toner due to the excessive current is caused at the portions other than the recess. Consequently, transfer defect is caused.
It is to be noted that, in the configuration disclosed in Japanese Patent Application Laid-Open No. 2012-42832, the transfer voltage and the returning voltage have different polarities, and therefore the change width between the transfer voltage and the returning voltage is large. Consequently, noise of undershooting of the voltage at falling of the pulse wave and noise of overshooting of the voltage at rising of the pulse wave have a large influence, and in turn, transfer defect is caused.